Schottky diodes are well known in the electronics industry. The popularity of their use primarily stems from the fact that they have a very low forward voltage drop and switching speeds that approach zero time making them ideal for a number of power applications. A prior art Schottky diode 5 is illustrated in FIG. 1A. As shown, a typical prior art Schottky diode includes a silicon substrate 10 that has an n-doped epi layer 12 formed thereon. Typically formed from the silicon substrate 10 is an isolation region 14, such as a field oxide region, and located on the epi layer 12 is a Schottky electrode 16, which is typically comprising titanium. Located between the electrode 16 and the isolation region 14 is an implanted p-type guard ring 18.
Unfortunately, however, due to a radius of curvature effects and process damages at the edges of the Schottky barrier, during operation, a field quickly builds up when a reversed bias is applied to the diode. This leads to a low breakdown voltage and poor leakage characteristics. Breakdown voltage has traditionally been improved by placing the diffused p-type guard ring 18 around the Schottky barrier, with the p guard ring short-circuited to the anode (metal plate) of the Schottky diode. This relieves the field, giving rise to a higher reversed bias voltage before substantial reversed current leakage occurs.
While, these prior art diodes have worked satisfactorily at lower reversed bias voltages, there has been an increase in the number of higher voltage output applications involving such diodes. For example, in telecommunication applications, the increase demand for high-speed Internet connections, such as DSL, has required an operating voltage in excess of 25 volts and in many cases in excess of 28 volts or greater. While operating in the forward bias direction, the Schottky diodes, as illustrated in FIG. 1A, perform at an acceptable voltage with little current leakage. However, in the reversed bias mode, these same diodes suffer from an unacceptably high amount of current leakage. They have either a constantly rising current leakage as the reversed bias voltage is increased or a sharp sudden increase in current leakage at high reversed bias voltages.
This effect is illustrated in FIG. 1B that shows a prior art Schottky diode. The graph line designated by reference number 20 shows a Schottky diode having a single implanted guard ring with a low doping concentration. As seen from this graph, the current leakage represented by the Y axis shows a steady increase as the reversed biased voltage, which is represented by the X axis increases. The graph line designated by reference number 22 shows a Schottky diode also having a single implanted guard ring but with a lower doping concentration. In this particular instance, the current leakage stays well below 1E-10 until the reversed bias voltage reaches about 24 volts. At that point, however, the current leakage increases exponentially to around 1E-04. Thus, as seen from this figure, either Schottky diode would be undesirable where the device is expected to operate at reversed biased voltages of 25 volts or more.
Accordingly, what is needed in the art is a diode that will operate at high reversed bias voltages without exhibiting the high current leakage as presently found in prior art diodes.